1. Field of the Invention
This invention relates to an optical system suitable for a great relative aperture long focus lens having the function of maintaining a sharp depicting performance for an object which is in focus (the main object) and yet varying the depictive property of out-of-focus of a long distance object (the background) outside the depth of field or a short distance object (the foreground) outside the depth of field.
Accordingly, the present invention basically differs in character from a conventional soft focus lens in which the amount of spherical aberration which occurs is varied to thereby vary the depictive property of the main object.
2. Related Background Art
When a cubic object has been photographed, an object off the depth of field is depicted as an out-of-focus image in the photograph, but generally, a photograph in which the edge portion of the out-of-focus image is clear-cut and two-line blur are not liked.
In contrast, to make the main object conspicuous, an out-of-focus image in which the edge portion in the defocus area is unclear and soft is also an important picture plane constituent, and the depictive property of this out-of-focus image is the subject of evaluation in the aesthetic viewpoint of photographs.
So, there are known numerous optical systems in which spherical aberration is intentionally created to realize low contrast depiction.
For example, a special optical element which is rotation-symmetrical with respect to the optic axis and in which the central portion is a parallel flat plate and the marginal portion has a refractive power or a certain transmittance characteristic and which is disposed in a lens system is disclosed in U.S. Pat. No. 4,013,347.
Also, optical systems in which the air space between lenses is varied to thereby realize low contrast depiction are disclosed in U.S. Pat. Nos. 4,124,276 and 4,310,221.
However, in U.S. Pat. No. 4,013,347 a special optical element is designed in a photo-taking lens system so as to be moved along the optic axis and therefore, the construction is complicated and this is not preferable.
Also, in U.S. Pat. Nos. 4,124,276 and 4,310,221, if the optical systems thereof are used for a great relative aperture long focus lens, a great driving force is required during focusing because they are of the focusing type in which the whole is axially moved, and it is difficult to effect quick focusing and further, there is the tendency of the lens barrel becoming complicated, and this is not preferable.
Also, there has heretofore been no lens for low contrast depiction which can obtain sharp depiction for the main object to which the lens is in focus and which realizes the depictive property in which, for the foreground or the background, the out-of-focus state is varied in conformity with the photographer's liking.